The present invention relates generally to optical systems, and more particularly, to optical digital coherent systems.
Multi-level (amplitude or phase) modulation formats, such as 8 PSK, 8 QAM or 16 QAM can be used to increase spectral efficiency. However, these signals with multi-level modulation need a high optical-signal-to-noise-ratio OSNR, which largely limits the transmission distance. Also, high optical-signal-to-noise-ratio OSNR transmitters are more complicated. Relative to these modulation formats, quadrature phase shift keying QPSK modulation systems have a simple configuration, but they have wide spectrum requirements at the same bit rate. Hence, the spectral efficiency for quadrature phase shift keying QPSK modulation systems is not high. Using optical sharp (tight) filter can increase the spectral efficiency of this signal, but this sharp filter usually degrades the transmission performance.
Accordingly, there is a need for increasing the spectral efficiency while maintaining good performance in a QPSK modulation system with a high optical-signal-to-noise-ratio OSNR.